1. Field of the Invention
Example embodiments relate to a method of determining a focus position for a substrate exposure process and a substrate exposure apparatus capable of performing the same. More particularly, example embodiments relate to a method of determining a focus position for an exposure process for transcribing image patterns onto a substrate such as a semiconductor wafer, for example, and an exposure apparatus capable of performing the same.
2. Description of the Related Art
In a fabrication process, electronic circuits may be formed on a semiconductor substrate such as a silicon wafer, for example. An electrical die sorting (EDS) process may be performed for inspecting electrical characteristics of elements and/or semiconductor devices formed on the semiconductor substrate. In a packaging process, semiconductor devices may be separated and packaged with epoxy resin.
The fabrication process may include a number of processes which are repeatedly performed to form electrical elements on the semiconductor substrate. For example, the fabrication process may include a deposition process, a photolithography process, an etching process, a chemical mechanical polishing process, an ion implantation process, a cleaning process, etc.
The photolithography process may be performed to form photoresist patterns on a layer formed by the deposition process. The photoresist patterns may be used as etching masks in the etching process so that the layer is formed into patterns having electrical characteristics.
The photolithography process may include a photoresist coating process for forming a photoresist layer on the semiconductor substrate, a baking process for hardening the photoresist layer, an exposure process for transcribing image patterns onto the photoresist layer, a development process for developing the photoresist layer to form the photoresist patterns, etc.
As semiconductor devices become highly integrated, sizes of patterns to be formed on the semiconductor substrate are reduced and thus, resolution becomes increasingly more important in the photolithography process. The resolution in the photolithography process may be influenced by a depth of focus (DOF), a focus position, etc.
Further, the resolution in the photolithography process may be influenced by a wavelength of an illumination light and/or a numerical aperture (NA) of a projection lens. Examples of the illumination light used in a photolithography process may include a g-line light beam having a wavelength of 436 nm, an i-line light beam having a wavelength of 365 nm, a krypton-fluoride (KrF) laser beam having a wavelength of 248 nm, an argon-fluoride (ArF) laser beam having a wavelength of 193 nm, and an F2 laser beam having a wavelength of 157 nm.
Increasing the numerical aperture of the projection lens to improve the resolution may cause the DOF to deteriorate. An off-axis illumination (OAI) may be used to improve the DOF by projecting zero-order and positive first-order light beams diffracted by image patterns of a reticle onto a semiconductor substrate.
Examples of the OAI may include an annular illumination, a dipole illumination, a quadrupole illumination, a cross-pole illumination, a hexapole illumination, etc.
Further, to increase the numerical aperture, various conventional techniques for improving focusing accuracy of a projected light passing through a reticle have been developed. However, as the integration degree of semiconductor devices increase, there still remains a need for an improved method of determining a best focus position for a substrate exposure process.